Re: [Xen-users] disk performance about half in domU? + question about XenSource

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I don't know how relevant our experience is since we are still on Xen
> 2.0.7 and just starting to test 3.1, however, we have noticed the same
> thing.

Yay - it's always somewhat satisfying to see how stable the 2.x series was for 
people and that they're still using it.  ISTR that 2.x (or maybe it was the 
early 3.x?) might have had some weird block performance regression that crept 
in.  The fact you're seeing it for 3.1 is a little weird though.

> We assumed it was simply I/O bottleneck - we have a single RAID
> controller trying to service several virtual servers.  We did dedicate a
> hyperthread to dom0 and changed the scheduler (I do not recall offhand
> to what) and those made significant improvements but we still cringe
> when we see how many CPU cycles are simply spent waiting for disk I/O.
> This is especially scary because, although it is a nasty mix of email,
> web and database servers along with intensive network I/O on two of the
> domUs which serve as VPN gateways, the actual usage is quite low - a
> handful of users coming across an Internet connection.  I don't know how
> these would fare under LAN load with a few hundred users.
>
> Not complaining - just sharing.  Thanks for a great product - John

Well, it sounds slower than I'd expect things to be from what I remember of 
the benchmarks...  Have you tried just doing a big dd (e.g. dd a big file 
into /dev/null using a decent block size) and seeing what raw bandwidth you 
can get?

Could you check what scheduler this is?  Credit is the most well supported at 
the moment.  Is it an SMP or a UP box?

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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